Add a couple functions required for yolo. (#527)

candle-nn/src/func.rs

@@ -0,0 +1,27 @@
+//! Layers defined by closures.
+use candle::{Result, Tensor};
+
+/// A layer defined by a simple closure.
+pub struct Func<'a> {
+    #[allow(clippy::type_complexity)]
+    f: Box<dyn 'a + Fn(&Tensor) -> Result<Tensor> + Send>,
+}
+
+impl<'a> std::fmt::Debug for Func<'a> {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "func")
+    }
+}
+
+pub fn func<'a, F>(f: F) -> Func<'a>
+where
+    F: 'a + Fn(&Tensor) -> Result<Tensor> + Send,
+{
+    Func { f: Box::new(f) }
+}
+
+impl<'a> super::Module for Func<'a> {
+    fn forward(&self, xs: &Tensor) -> Result<Tensor> {
+        (*self.f)(xs)
+    }
+}